1. Field of the Invention
The present invention relates to a lithographic apparatus, a method for manufacturing a device, and a device manufactured therewith.
2. Background of the Related Art
A lithographic apparatus is a machine that applies a pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of one or several dies) of a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction, to be discussed in more detail below) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Imprinting may be carried out in a step-like way, wherein a stationary substrate is imprinted with a pattern from a stationary patterning device. An alternative way comprises scanning the substrate with a patterned beam. One of the known ways to do this is by way of pulsed illumination of the patterning device, and thus of the substrate. The image on the substrate may thus be built up of many pulsed illuminations of the patterning device, in such a way that the pulsed images overlap locally, and a sharp image is formed. This scanning type of illumination will be discussed more extensively below.
A problem with scanning type of illumination is that imperfections of the optical system may be imaged onto the substrate. These imperfections relate to those that cause the intensity, and thus the total received dose, as a function of position in the slit to be non-homogeneous, and in particular to those that extend across the radiation beam in the scanning direction. Such imperfections may be in the form of dust particles on an illuminator lens, inhomogeneities of the optical lens material, and in particular, e.g., reflections in an optional beam delivery system or other optical element such as a quartz rod or calcium fluoride rod, etc. A quartz (or calcium fluoride, etc.) rod with a square diameter is sometimes used to homogenize the supplied beam of radiation. The side faces and edges of the rod, and reflections thereof, may cause inhomogeneities in the radiation beam. Especially, but not only, imperfections in the radiation beam with a dimension in scanning direction of more than the distance over which the patterning device is displaced between two consecutive pulses would cause an overlap of their respective inhomogeneities, and thus a more visible straight line, although other types of imperfections causing stripes are not excluded.
Especially in the case of illuminating a substrate without using a patterning device, this may become visible in the form of stripes. Of course, in most practical cases, a patterning device will be used, but still the underlying basic radiation intensity will show such stripes, which may become visible on the actual substrate as a straight line of feature irregularities, parallel to the scanning direction.
In the prior art, this disadvantage has been reduced by adjusting the intensity profile by defocusing the illumination system, in order to smear out irregularities. However, this known measure does have disadvantages. For example, defocusing the illumination system causes the masking blades for the patterning device to be out-of-focus. Such masking blades are used to delimit the illumination field in order to prevent illumination of unwanted parts of the patterning device or substrate and to prevent scattered radiation. The masking blades thus define a so-called black border around the illuminated part of the patterning device. Because in practice the radiation in the radiation beam has a large angular distribution, these masking blades should be positioned close to the patterning device in order to not loose too much radiation. The masking blades thus being out of focus means that the definition of the black border is undesirably affected. Another disadvantage is that irregularities caused by the exposure lens, i.e., the lens between the patterning device and the substrate, cannot be reduced by this measure.